The processes involved in the regulation of gene expression in eucaryotic organisms are not well understood. In Drosphila the final expression of enzymatic activity for three molybdenum hydroxylase enzymes, XDH, AO and PO, is under control of at least three nonstructural loci, ma-1, cin and lxd. Although the exact genetic and biochemical basis for such control is unclear, the available evidence indicates that these loci are involved in post-translational processing or the synthesis/acquisition of a molybdenum cofactor shared by these enzymes. We propose to examine possible post-translational modifications by using immunoprecipitation and two-dimensional gel electrophoresis to characterize the hydroxylase polypeptides present in both wild type and mutant strains. These polypeptides will also be analyzed for the presence of the molybdenum cofactor. We will extend our previous genetic analysis of these loci to include intragenic mapping of two of them, as well as the isolation of a wider variety of alleles. We also intend to identify loci affecting other processes involved in the expression of XDH, AO and PO. After a careful biochemical analysis of the events involved in the developmental expression of these enzymes in wild type flies, we intend to isolate and characterize variants in which the normal developmental or tissue specific pattern of enzyme expression is altered.